Ventilation support is needed when patient cannot meet the gas exchange demand with his own respiratory action. Such situations are typically during intensive care and surgical anesthesia. Typical reasons for inability to maintain the gas exchange demand may be sedation for therapeutic reasons including neural dampening and muscular relaxation and muscular weakening due to underlying disease. The ventilation support may be used to enhance carbon dioxide (CO2) clearance and oxygen delivery to the patient. Further purpose of ventilation is delivery of inhalation anesthesia gas when ventilation is used during inhalation anesthesia.
Ventilation support divides into two categories: full mechanical ventilation and spontaneous breathing support, or pressure support. In mechanical ventilation, the ventilator dictates the breath rate and volume and is necessary where, for example, muscle relaxants are administered. By contrast, in spontaneous breathing support the patient maintains the respiratory rhythm and the ventilator is controlled to detect inspiration breaths. In spontaneous support ventilation, the ventilator adds inspiration pressure as a response to patient generated spontaneous breath.
Support of patient generated spontaneous breathing with added ventilation pressure is preferred ventilation on intensive care and also increasingly during anesthesia whenever the surgery does not require complete relaxation. The rationale for its use is to maintain patient's muscular activity and ease the weaning from ventilation.
In presently-available systems for spontaneous breathing support, the clinician controls the amount of ventilation to maintain appropriate subject's CO2 concentration or partial pressure. This level can be determined by analysis of blood sample for arterial blood CO2 partial pressure, PaCO2. However, because this is a discrete measurement, end-expiratory gas CO2 (EtCO2) concentration is often used as surrogate for this. A typical EtCO2 value is around 5%-6%, or 5-6 kPa, but in certain circumstances the optimum value may deviate from this.
Patient metabolic CO2 production varies between subjects. This depends e.g. on subject size, age, gender, anxiety level, sedation, etc. Also some treatment actions vary the CO2 level. To maintain the optimal subject CO2 level the amount of ventilation must be adjusted to meet the CO2 clearance demand. Furthermore, during mechanical ventilation this amount of ventilation must be divided between the breath rate and volume optimizing between minimal lung pressure and total amount of ventilation.